Optical spectrometers are widely used when the composition of a gas or a fluid must be measured. Such measurements are needed in industry, science, medicine, pharmacy, agriculture and biology in a wide range of applications. Infrared emitters play an important role in the development of highly accurate chemical sensors, and serve as the principal infrared emitting element in optical gas sensor systems. Conventionally an IR (infrared) emitter has taken the form of a wire, filament or conductive ceramic element heated by electrical current. The infrared emission is dependent on temperature and area of the heated surface. The demand for small-sized spectrometers for portable measuring devices and system integrated sensors has resulted in increasing interest in microspectrometers. Some commercial solutions are based on simple thin film emitters of, for example, tungsten or platinum. However, emissivity of tungsten or platinum is low.
Optically black surfaces can be used either as an absorber or an emitter. Absorbers are needed in thermal detectors and emitters in thermal infrared sources. The former do not need to withstand as high temperatures as the latter.
Emissivity of a simple metal surface can be enhanced multifold by adding properly tailored thin film layer structure on top of the metal. In order to be able to use the optical structure as an emitter, the structure must withstand the temperature required in emitter use. Typically, in emitter applications temperatures of more than 100° C. are used, which the structure should preferably withstand unchanged for even long periods of time. The operating temperature can also be higher, for example at least 200° C., at least 300° C., or even more than 650° C. Emitter temperatures most usually used are in the range 100-1000° C., for example in the range 200-650° C., such as in the range 250-400° C. Thus, the structure should withstand the design temperature for the application and remain stable in the selected operating temperature range for very long periods of time. At the same time, the structure should permit a good optical matching. Stability means that the emissivity of the structure remains essentially unchanged at the desired wavelength range in the operating conditions of the structure for the duration of the operating life of the structure.
Manufacturing of the layered structure suitable for emitter use is, therefore, very demanding, because the effective emitter use of the component requires a high temperature. For this reason, most of the known absorber structures are not in practice suitable for emitter use.
Absorber structure presented by Liddiard K.C. (Infrared Physics, 1993, vol. 34, 4, pp. 379) cannot be used as infrared emitter because it will not tolerate the temperatures demanded in emitter use. In Liddiard's solution a thin semi-transparent metal layer and a dielectric layer are on top of non-transparent metal layer. The thin metal layer together with the dielectric layer forms an antireflective layer structure so that reflection from the surface is very low at desired wavelength range. Thus with low reflection and zero transmission, high absorption efficiency can be obtained.
U.S. Pat. No. 6,177,673 discloses an infrared absorber which is based on use of doped silicon and non-transparent metal layer. This kind of structure is not optically stable in emitter use. The main reason for the optical instability is the activation level of dopants (solid solubility) that is dependent on temperature.
US2015241612 discloses an infrared emitter structure which is based on the same operating principle as the Liddiard's absorber. A thin metal layer and an underlying dielectric layer are used as antireflective layers on top of a non-transparent reflective metal layer. Optical stability is achieved by protecting the thin lossy metal layer with one or more shielding layers.
The article “Thin film absorbers for visible, near-infrared, and short-wavelength infrared spectra”, Proceedings of the Eurosensors XXIII conference, Procedia Chemistry Volume 1, Issue 1, September 2009, Pages 393-396, discloses an absorber structure which is basically the same structure as the emitter structure presented in US 2015241612.
Stable light sources with high emission efficiency are not available in the market. Present commercial solutions are expensive, and either of low efficiency and large power consumption, or not sold as discrete light-emitting devices.
There is a need for low cost, low power consumption, stable infrared source, especially for spectroscopic and NDIR gas, liquid and solid matter measurement applications.